[SQL] remove unnecessary import in spark-sql

Author: Jacky Li <jacky.likun@huawei.com>

Closes #3630 from jackylk/remove and squashes the following commits:

150e7e0 [Jacky Li] remove unnecessary import

sql/core/src/main/scala/org/apache/spark/sql/SchemaRDD.scala

@@ -18,7 +18,7 @@
 package org.apache.spark.sql
 
 import java.util.{Map => JMap, List => JList}
-import java.io.StringWriter
+
 
 import scala.collection.JavaConversions._
 

sql/core/src/main/scala/org/apache/spark/sql/execution/ExistingRDD.scala

@@ -19,13 +19,11 @@ package org.apache.spark.sql.execution
 
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.rdd.RDD
-import org.apache.spark.sql.{DataType, StructType, Row, SQLContext}
+import org.apache.spark.sql.{StructType, Row, SQLContext}
 import org.apache.spark.sql.catalyst.ScalaReflection
-import org.apache.spark.sql.catalyst.ScalaReflection.Schema
 import org.apache.spark.sql.catalyst.analysis.MultiInstanceRelation
 import org.apache.spark.sql.catalyst.expressions.{Attribute, GenericMutableRow}
 import org.apache.spark.sql.catalyst.plans.logical.{LogicalPlan, Statistics}
-import org.apache.spark.sql.catalyst.types.UserDefinedType
 
 /**
  * :: DeveloperApi ::

sql/core/src/main/scala/org/apache/spark/sql/execution/GeneratedAggregate.scala

@@ -18,7 +18,6 @@
 package org.apache.spark.sql.execution
 
 import org.apache.spark.annotation.DeveloperApi
-import org.apache.spark.sql.SQLContext
 import org.apache.spark.sql.catalyst.trees._
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.plans.physical._

sql/core/src/main/scala/org/apache/spark/sql/execution/SparkPlan.scala

@@ -22,14 +22,11 @@ import org.apache.spark.Logging
 import org.apache.spark.rdd.RDD
 import org.apache.spark.sql.SQLContext
 import org.apache.spark.sql.catalyst.{ScalaReflection, trees}
-import org.apache.spark.sql.catalyst.analysis.MultiInstanceRelation
 import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.catalyst.expressions.codegen._
 import org.apache.spark.sql.catalyst.plans.QueryPlan
-import org.apache.spark.sql.catalyst.plans.logical.LogicalPlan
 import org.apache.spark.sql.catalyst.plans.physical._
 
-
 object SparkPlan {
   protected[sql] val currentContext = new ThreadLocal[SQLContext]()
 }

sql/core/src/main/scala/org/apache/spark/sql/types/util/DataTypeConversions.scala

@@ -23,7 +23,7 @@ import scala.collection.JavaConverters._
 
 import org.apache.spark.sql._
 import org.apache.spark.sql.api.java.{DataType => JDataType, StructField => JStructField,
-  MetadataBuilder => JMetaDataBuilder, UDTWrappers, JavaToScalaUDTWrapper}
+  MetadataBuilder => JMetaDataBuilder, UDTWrappers}
 import org.apache.spark.sql.api.java.{DecimalType => JDecimalType}
 import org.apache.spark.sql.catalyst.types.decimal.Decimal
 import org.apache.spark.sql.catalyst.ScalaReflection